1. Field of the Invention
The present invention relates to an acceleration sensor with at least one magnetoresistive effect (MR) element and to a magnetic disk drive apparatus with the acceleration sensor.
2. Description of the Related Art
There are magnetic disk drive apparatuses or hard disk drive (HDD) apparatuses assembled in mobile equipments such as for example walkabout personal computers, mobile phones and other mobile gears, provided with shock sensors. The shock sensor generates a signal for immediately interrupting write current when a drop impact is applied to the HDD apparatus so as to avoid corruption of data written in a magnetic recording medium or a hard disk.
The shock sensor is used to merely detect whether a certain impact is applied or not. Therefore, corruption of the written data due to the drop impact can be prevented to some extent, but it is impossible to prevent a collision of a magnetic head with the hard disk surface due to the drop impact and thus damage of the hard disk and breakage of the magnetic head may be induced.
In order to reliably prevent both corruption of data and damages of the magnetic head and the hard disk, it is necessary to detect the instant at which the HDD apparatus falls before occurrence of drop impact and to retract the magnetic head from the hard disk surface. Such instant of the falling can be detected from a change in the acceleration of gravity.
Japanese patent publication No. 02-248867A discloses a piezo-electric type acceleration sensor for detecting a small change in the acceleration of gravity from a change in stress of springs. This sensor has springs in dual tuning fork vibrators, a weight supported by the springs, and piezo-electric elements attached on the springs to detect the change in stress applied to the springs from the weight.
U.S. Pat. No. 5,747,991 discloses an electrostatic capacitance type acceleration sensor for detecting a small change in the acceleration of gravity from a displacement of a weight. This sensor has a movable electrode and a static electrode faced to each other to detect a change in electrostatic capacitance from a change in distance between the movable and static electrodes due to the acceleration.
Such known piezo-electric type acceleration sensor or electrostatic capacitance type acceleration sensor needs to have electrodes for extracting detection signals there from on the spring or the weight attached to the spring and also lead lines electrically connected to the electrodes. Thus, the structure of the sensor becomes complicated due to the lead lines connected to the electrodes. Also, when the spring and weight are miniaturized, wiring process of such lead lines becomes extremely difficult. Further, the lead lines formed on the miniaturized spring or weight may induce breakage of the spring when an excessive value of impact is applied, and prevent movement of the spring to interface with the improvement in sensitivity of the sensor. This tendency becomes more pronounced as the acceleration sensor becomes smaller.
U.S. Pat. No. 6,131,457 discloses an acceleration sensor that may solve the above-mentioned problems in the conventional piezo-electric type acceleration sensor and electrostatic capacitance type acceleration sensor. This acceleration sensor has a magnetic body including a mass point, mounted to a vibrator having three-dimensional freedom and an axis in line with a Z-axis, and four or more detector MR elements positioned on an X-axis and a Y-axis with their centers located along a perimeter of a concentric circle around the origin point of the orthogonal coordinate axes. The sensor is thus capable of detecting each of acceleration in the direction of X-axis through a relative difference in output voltage between the two detector elements on the X-axis due to a vibration of the magnetic field, acceleration in the direction of Y-axis through a relative difference in output voltage between the two detector elements on the Y-axis due to a vibration of the magnetic field, and acceleration in the direction of Z-axis through a sum total of the output voltages of all the detector elements.
According to the acceleration sensor disclosed in U.S. Pat. No. 6,131,457, as it is not necessary to form electrodes on the spring nor the weight, the structure of the sensor becomes simple. However, because an anisotropic MR (AMR) element with a single layer structure of an MR material is used as for each magnetic field detector element, two or more detector elements are necessary for detecting acceleration in a direction along a single axis. Thus, this acceleration sensor requires the great number of detector elements causing the miniaturization of the sensor itself to become extremely difficult. Also, wiring of the many detector elements induces complicated structure of the sensor. Furthermore, using of the AMR elements causes lower sensitivity of the magnetic field, so that it is difficult to provide a highly sensitive acceleration sensor.